The present invention relates to a lead acid storage battery especially used for automobiles.
In lead acid storage batteries for automobiles, either one of a positive electrode plate and a negative electrode plate, which are obtained respectively by filling a grid with a paste of active material is accommodated in a bag-like separator that is composed of a fine porous synthetic resin sheet A plurality of vertical ribs are formed in parallel to one another on an inner surface of the bag-like separator and are arranged to extend along a vertical axis of the electrode plate. This arrangement causes a certain quantity of electrolyte to be present around the electrode plate in the bag-like separator, and prevents the inner surface of the separator from being in direct contact with the surface of the electrode plate received in the separator.
The bag-like separator separates the positive electrode plate from the negative electrode plate and causes the active material falling off the grid an the course of the use of the storage battery to be reserved in the separator bag, thereby preventing an electric short circuit between the positive electrode and the negative electrode.
The active material of the positive electrode plate is readily and significantly softened and falls off the grid by the repeated charge and discharge, compared with the active material of the negative electrode plate. The general configuration thus accommodates the positive electrode plate in the bag-like separator. The bag-like separator in which the positive electrode plate is accommodated, advantageously interferes with the fall-off of the active material of the positive electrode even if the active material is softened to some extent.
With a recent trend of the high-performance engine and the compact body, the engine room in automobiles tends to be exposed to extremely high temperatures. When the storage battery is used at high temperatures, the corrosion of the grid of the positive electrode plate proceeds to worsen the cycle life of the storage battery especially when an antimony-free lead-calcium-tin alloy is used for the grid of the positive electrode plate and the negative electrode plate in order to prevent a decrease in volume of the electrolyte under the high temperature conditions, the corrosion of the grid of the positive electrode plate is prominent and significantly worsens the cycle life of the storage battery
One proposed technique to prevent the cycle life of the battery from being worsened enhances the strength of the grid For example, the technique disclosed in Japanese Laid-open Patent No. Hei 5-290857 raises the concentration of tin in the lead-calcium-tin alloy, which forms an expanded grid of the positive electrode plate, thereby ensuring the sufficient strength of the grid and improving the cycle life of the storage battery. The expanded grid of the positive electrode plate reduces the degree of corrosion and improves the life cycle of the battery. The positive electrode plate of the expanded grid accommodated in the bag-like separator, however, may break the bottom of the bag-like separator to cause a short circuit between the positive electrode and the negative electrode and abruptly worsen the cycle life of the battery. The expanded grid has a lower degree of corrosion than the conventional grid, but expands in a specific expanding work direction due to the corrosion. This causes the bottom of the separator to be damaged. The structure that applies an expanded grid for the positive electrode plate and accommodates the positive electrode plate in the bag-like separator by taking into account the possible corrosion of the grid under the high temperature condition is an effective countermeasure to enhance the strength of the grid. The problem of this technique is that the expansion of the expanded grid in the specific expanding work direction damages the separator and thereby abruptly worsens the life cycle of the battery.
One proposed technique to solve this problem accommodates the negative electrode plate in the bag-like separator, instead of the positive electrode plate made of the expanded grid. This arrangement prevents the positive electrode plate from going the bottom of the bag-like separator, but causes another problem, that is, fall-off of the active material from the expanded grid of the positive electrode plate. In the conventional structure, the softened active material of the positive electrode falls off the grid little by little. In this proposed structure, on the other hand, the active material of the positive electrode included in one lattice falls off at once on both sides of the positive electrode plate. This is ascribed to the characteristics that the expanded grid of the positive electrode plate expands in the specific expanding work direction and that the expanded grid does not have the framework on both the left and right sides thereof. These characteristics do not cause any significant problem in the conventional structure where the positive electrode plate is accommodated in the bag-like separator. In the proposed lead acid storage battery where the negative electrode plate, instead of the positive electrode plate, is accommodated in the bag-like separator, however, the fall-off of the active material significantly worsens the life cycle of the battery. This phenomenon is especially prominent under the high vibrating conditions, for example, in recreational vehicles widely used.
The object of the present invention is thus to prevent an active material from falling off a positive electrode plate and to improve the life cycle of a lead acid storage battery under high vibrating conditions, where the lead acid storage battery has the positive electrode plate including an expanded grid and a negative electrode plate accommodated in a bag-like separator.
The present invention is directed to a lead acid storage battery comprising an assembly element which comprises a plurality of positive electrode plates and negative electrode plates that are stacked alternately, each negative electrode plate being accommodated in a bag-like separator, wherein the separator is provided by folding a fine porous synthetic resin sheet and sealing left and right overlapping sides of the folded sheet to have a bag-like shape the separator having a plurality of vertical ribs that are formed in parallel to one another on an outer surface of the separator and that are located in a central portion of the separator occupying a most part of its width, the separator further having small rib areas that extend along a length of the bag-like separator and that are arranged on left and right sides thereof, each of the small rib areas including a large number of small ribs that intersect left or right side end of the positive electrode plate.
In accordance with one preferable mode of the present invention, the positive electrode plate comprises a grid which""substantially does not have a vertical framework, and a paste of active material applied to the grid.
It is preferable that a lattice width of the grid is not greater than 1.4 times a width of the each small rib area.
In accordance with another preferable mode of the present invention, some or all ends of the small ribs on a center side of the separator are continuous with an adjoining vertical rib;
In accordance with still another preferable mode of the present invention, each of the small ribs is arranged to have its longitudinal axis inclined to the horizontal direction.
In accordance with another preferable mode of the present invention, the positive electrode plate comprises an expanded grid of an lead-calcium-tin alloy and a paste of active material applied to the expanded grid.
It is preferable that a content of tin in the lead-calcium-tin alloy ranges from 0.7 to 2.2% by weight.
It is also preferable that a content of calcium in the lead-calcium-tin alloy ranges from 0.05 to 0.09% by weight.